Lead neurotoxicity and socioeconomic status: conceptual and analytical issues

Socioeconomic status (SES) is usually considered to be a potential confounder of the association between lead exposure and children's neurodevelopment, but experimental and epidemiological data suggest that SES might also modify lead neurotoxicity. The basis of this effect modification is uncertain, but might include differences among SES strata in co-exposures to other neurotoxicants, genetic susceptibilities, environmental enrichment, and stress. The role of SES in the causal nexus is likely to include other dimensions, however. It conveys information about lead exposure opportunities as well as about predictors of child outcome that are correlated with but causally independent of lead. Failure to distinguish these aspects of SES will lead to an underestimate of lead's contribution, and might even result in attributing to SES health effects that should be attributed to lead. Conventional models, which treat SES and SES-related factors solely as potential confounders, do not capture the possibility that a child's early lead exposure alters the behaviors that the child elicits from others. Failure to model lead's contribution to such time-varying covariates will also tend to bias estimates of lead neurotoxicity toward the null. On a trans-generational level, low SES might be a proxy for vulnerability to lead. To estimate the burden of lead-associated neurotoxicity on a population level, we need to apply analytical approaches that model a child's development and its context as a complex system of interdependent relationships that change over time.